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The sip:refer verb is used to transfer a call and disconnect it. This verb sends a REFER message to the carrier (SIP trunk) or SIP phone. For the transfer to be successful, the carrier or SIP phone needs to support the REFER message. Once the sip:refer is successfully completed, the call will be transferred and disconnected from Voice Gateway. 
{
  "verb": "sip:refer",
  "referTo": "+49XXXXXXXXXXX",
  "actionHook": "/action"
}

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Configuration

The full set of configuration parameters:
ParametersDescriptionRequired
referToA SIP URI or a phone number/user identifier.Yes
referredByA SIP URI or a phone number/user identifier. If the value is not provided, it will default to the identity of the party being transferred.No
actionHookA webhook to receive an HTTP POST when the call was completed. Default is referNo
eventHookA webhook to receive an HTTP POST when NOTIFY messages of follow-on call status are received.No
headersAdditional SIP headers to include in the response.No
The sip:refer verb completes when one of the following conditions is met:
  • A failure response is received to the REFER.
  • A 202 Accepted response is received in response to the REFER, and a NOTIFY of the follow-on call status with a final call status is received.
  • The sip:refer has an action hook that will provide details of the final result, as well as an event hook that is called for every NOTIFY received after a successful REFER.

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actionHook

The actionHook webhook contains the following additional parameters:
ParameterDescription
referStatusThe SIP status response to the REFER request.
final_referred_call_statusThe parameter indicates the final SIP status of the subsequent call to the transferee. This parameter is provided only when the REFER is accepted and NOTIFY requests are received from the remote party.
eventHook.eventTransfer status.
eventHook.call_statusContains a SIP status received in a NOTIFY after a successful REFER.
Example: 
{
	"event": "transfer-status",
	"call_status": "180"
}